<!DOCTYPE html>
<head>
    <script src=""></script>
    <!--  <script>
    alert(123)</script>    
    -->
    <script src="demo.js"></script>
</head>
<body>
    今天
</body>
<script>
    alert(789)
    /* 
    alert是弹框命令
    */
   //  //是单行注释  /* */ 是多行注释
</script>